The overall objective of this project is to study the immune mechanisms involved in the induction and progression of rat leukemia. It was recently discovered in our laboratory that passage of leukemic cells through x-irradiated recipients is followed by the permanent loss of antigenic expression with subsequent changes in transplantability and invasiveness. This new method for deliberate induction of antigenic modulation will be used to study the alterations that occur during the process in both the leukemic cell and the host. Sera of irradiated rats will be added to cultures of leukemic cells to attempt the induction of antigenic modulation in vitro. Similarly to explore the role of humoral antibodies in the process of antigenic deletion, leukemic cells will be placed in Millipore diffusion chambers that will be implanted i.p. in irradiated rats. Reconstitution of irradiated rats with normal lymphocytes of thymus and/or spleen will be also attempted.